msm: npu: Apply thermal mitigation properly

If thermal mitigation is requested when NPU is OFF, it should be
applied after the NPU is turned ON. When NPU is powered off, the
previous power level from thermal mitigation should be retained.

Change-Id: I58c56687d20a67f370f1dc2658fcb80fca3eb244
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>